1. Field of the Invention
This invention relates to composite coatings which are resistant to both corrosion and ultraviolet (xe2x80x9cUVxe2x80x9d) radiation for exterior and interior components, such as transformers, circuit breakers, and the like, with their associated housings used in electrical equipment, particularly for outdoor electrical equipment and associated components.
2. Background Information
Protective coatings for electrical equipment are well known in the art, and taught, for example, by U.S. Pat. Nos. 3,979,704 and 4,298,656 (Buckely et al. and Mendelsohn, respectively), the former relating to a composite coating, with a rough zinc or iron phosphate layer covered by a zinc chromate or dichromate layer for metallic contacts, and sensing, tripping and supporting circuit breaker members. The latter patent relates to sprayable, flexible, crack-resistant, adhesive bracing compositions, for generator stator end windings, where the compositions are made from a mixture of bisphenol A epoxy resin, butadiene/acrylonitrile polymer, coloring pigment, thixotropic agent, and curing agent.
In other areas, U.S. Pat. Nos. 5,178,902 and 5,300,336 (both Wong et al.) teach protective coatings for metal pipes, the coating having an epoxy resin primer layer next to the pipe surface, a polyolefin (polyethylene, polypropylene) exterior sheath, and an interlayer mixture of epoxy and polyolefin. In the application process, the pipe is surface blast cleaned, washed to remove metallic dust and heated to between 175xc2x0 C. and 275xc2x0 C., then the three layers are applied in a single electrostatic powder application booth where the resin particles fuse bond to each other. Post heating can also be utilized followed by a water quench.
A series of brochures by 3M: 3M(trademark) Scotchkote(trademark) Fusion Bonded Epoxy Coatings (2000), pp. 1-11; 3M Scotchkote(trademark) 134 Fusion Bonded Epoxy Coating (1999), pp. 1-4; 3M Scotchkote(trademark) 134 Fusion Bonded Epoxy Coating-Information, Properties and Test Results, (2000), pp. 1-12; and 3M Scotchkote(trademark) 134/135 Fusion Bonded Epoxy Coating (2000), pp. 1-4, disclose epoxy powder coating compositions which offer corrosion resistance protection to metals and which can be applied by fluidized bed, air spray, or electrostatic spray techniques which can be used over Scotchkote(trademark) liquid phenolic resin primer and which can be over-coated with other materials for abrasion resistance, UV protection and impact protection via a cellular structure. These components may comprise epoxy resin, curing agent, pigments, catalysts, filler, and flow control agents uniformly mixed into each discrete particle. These coatings can be applied to piping, pump housings, valves, flow meters, ladders, wire mesh, and rebar rods, among other articles.
Multilayer polyolefin systems containing a base fusion bonded epoxy layer, a polyethylene or polypropylene adhesive intermediate layer and a polyethylene or polypropylene topcoat are also described. General application steps are removal of oil or grease, abrasive blast clean, pre-heat, deposit the fusion bonded epoxy powder, cure by heating, and a final inspection. For internal pipe coating a liquid epoxy primer is applied after abrasive blast cleaning. To add color the finished product can be coated with alkyd paint, acrylic lacquer or acrylic enamel.
While many epoxy coatings provide excellent corrosion resistance, and in many instances, in order to provide superior long-term corrosion resistance, stainless steel is used adding substantially to costs, what is needed in the industry is an inexpensive composite coating with even more enhanced, long-range corrosion resistance for extreme outdoor conditions, which coating will also provide excellent UV resistance, and which can also be used for interior applications.
Therefore, it is a main object of this invention to provide an article and process involving composite coating metal articles, usually galvanized steel, to provide superior toughness and weatherability and excellent UV resistance, eliminating the need to use expensive stainless steel components. It is another main object of this invention to provide an article and process involving composite coating steel or other articles to provide corrosion resistance for internal and external parts used in electrical equipment and associated components but not expected to carry current.
These and other objects are met by providing an article suitable for use in or to contain electrical equipment comprising: a metal article having successive coating layers of an inner layer of phosphate; non-chrome sealant effective to fill pores in the phosphate layer; thermoset, filled, epoxy resin; phosphate; non-chrome sealant effective to fill pores in the phosphate layer; thermoset, filled, epoxy resin; and an outer layer of pigment-containing paint resistant to ultraviolet rays.
Preferably, total application of all the coating layers is from about 10 to 30 milligrams per square foot (0.9 to 2.8 milligrams per sq. meter), with the outer paint layer being a polyester/polyurethane paint having a thickness of from about 0.030 mm to about 0.090 mm. Preferably, the metal article is galvanized steel.
The invention also resides in a method of coating a metal article comprising: (a) cleaning the metal article with an alkali hydroxide having a pH of at least 12; and then (b) coating with a heated aqueous phosphate solution having a pH of from about 4 to 6; and then (c) coating with a non-chrome sealant having a pH of from about 2.5 to 3.5; and then (d) drying the sealant to fill pores in the phosphate coating; and then (e) heating the coated metal member up to about 150xc2x0 C. to 275xc2x0 C.; and then (f) fuse bond coating the coated, heated metal member with a 100% solids, thermoset, filled, epoxy resin; and finally (g) painting the coated metal member with a pigment containing ultraviolet ray resistant paint.
Preferably, steps (a) through (f) are repeated before final painting in step (g) and the steel member is washed with water between steps (a) and (b), and between steps (b) and (c). Preferably, the metal member is steel, galvanized prior to any coating and the paint is a polyester/polyurethane paint.
This provides a thick extremely durable, totally corrosion resistant, UV resistant coated article for indoor or outdoor use for manufactured parts, which may be stamped, welded or machined prior to treating and coating. The process mostly uses solventless resins presenting minimal hazardous off-gases. The process also conforms to Underwriters Laboratories standards for safety xe2x80x9c1995-UL-1332 Organic Coatings for Steel Enclosures for Outdoor Use Electrical Equipmentxe2x80x9d for exposure to salt spray, moist carbon dioxide/sulfur dioxide, and light/water, and the like.